<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>3</title>
</head>
<style>
  .container{
    display: flex;
  }
  .box{
    height: 800px;
  }
  #div1{
    background: red;
    width: 200px;
  }
  #div2{
    background: blue;
    flex-grow: 1;
  }
  #div3{
    background: yellow;
    width: 200px;
    align-self: flex-end;
  }
</style>
<body>
  <div class="container">
    <div class="box" id="div1"></div>
    <div class="box" id="div2"></div>
    <div class="box" id="div3"></div>
  </div>
</body>
</html>